The present invention relates to a speed change ratio control unit for a V-belt type continuously variable transmission and a toroidal type of continuously variable transmission (in this specification, these are named generically and only referred to as “continuously variable transmission”). More particularly, in a continuously variable transmission using a step motor as the driving actuator of a speed control valve, the invention relates to a speed change ratio control unit which provides preventive measures against step-out of the concerned step motor.
In the case of a V-belt type continuously variable transmission, by hydraulically changing the diameter ratio between a pair of pulleys (a drive pulley and a driven pulley) across which a V-belt is laid; in the case of a toroidal type continuously variable transmission, by inclining power rollers hydraulically to change the contact diameter between the input and output disks, and in both cases, the speed change ratio of a vehicle such as an automobile is changed in a non-step manner.
In these continuously variable transmissions, a hydraulic pressure valve called change gear control valve is provided in order to change the diameter ratio between the pair of pulleys or in order to change the inclination amount of the power rollers. Such continuously variable transmissions which use a step motor as a driving actuator of the speed change control valve have been known.
A step motor is superior in obtaining accurate rotation angle (step position) corresponding to the number of control pulses. However, the synchronization between the control pulse and the motor revolution is sometimes lost in such cases when a sharp speed change is made or an overload is applied. Such situation described above is called “step-out.”
In the continuously variable transmissions using a step motor (also, called a stepping motor) for the driving actuator of the speed change control valve, a conventional type speed change ratio control unit equipped with step-out preventive measures for the step motor is known. For example, when a switch (low switch) is provided which turns on when a spool of a speed change control valve reaches to a specific position corresponding to the maximum speed change ratio (position of maximum speed change ratio); determining whether the low switch is on or not at every predetermined control cycle; and when it is on a value of the step position (ASTP) of the step motor obtained by the operation unit is replaced with a predetermined value (LOWSTP) corresponding to the above maximum speed change ratio. This unit is described in, for example, Japanese Laid-open (Kokai) Patent Application (A) numbered Heisei 8-114260 (1996) titled “CHANGE GEAR RATIO CONTROLLER OF CONTINUOUSLY VARIABLE TRANSMISSION.”
According to the above control unit, when the spool of the speed change control valve is at a specific position corresponding to the maximum speed change ratio, the low switch is turned on. Since the ASTP is replaced with the LOWSTP, a condition of “ASTP≠LOWSTP” due to a step-out can be corrected to a condition of “ASTP=LOWSTP”